In order to mount some component, structure, or the like on a drilling object such as a concrete wall, the following technique is widely used: fix an anchor bolt in a bottomed mounting hole formed in the concrete wall surface; and fit the component, structure, or the like to the anchor bolt. The mounting hole includes a thin hole portion and an expanded-diameter hole portion. The thin hole portion is open at the wall surface, and the expanded-diameter hole portion is formed at the inner bottom part of the thin hole portion. The anchor bolt has an expanded-diameter head portion, and the head portion is fitted in the expanded-diameter hole portion.
Various devices for drilling such a mounting hole have been proposed. The applicant of the present application also discloses one of such devices in Patent Literature 1. The device disclosed in Patent Literature 1 is configured as follows: tapered surfaces are formed on the circumferential surface of an expanded-diameter guide provided at the distal end portion of the device; and expanded-diameter bits are slidably fitted to the expanded-diameter guide. After the expanded-diameter guide is inserted in a thin hole, the expanded-diameter bits in a rotating state are pushed in toward the inner part of the thin hole. While being pushed in, the expanded-diameter bits spread out laterally along the tapered surfaces, and the expanded-diameter bits form an expanded-diameter hole portion with their rotating motion at the inner part of the thin hole.